Relighting a turbofan engine

ABSTRACT

The method and apparatus for in-flight relighting of a turbofan engine involve in one aspect selectively controlling an accessory drag load on one or more windmilling rotors to permit control of the windmill speed to an optimum value for relight conditions.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

The present application is a divisional of U.S. patent application Ser.No. 11/379,622 filed Apr. 21, 2006, now U.S. Pat. No. 7,861,533 theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to the in-flight relighting of an aircraftturbofan engine.

BACKGROUND

FIG. 1 schematically illustrates a typical turbofan engine 10 forsubsonic flight. The engine 10 generally comprises in serial flowcommunication a fan 12 through which ambient air is propelled, amulti-stage compressor 14 for pressurising the air, a combustor 16 inwhich the compressed air is mixed with fuel and ignited for generating astream of hot combustion gases, and a turbine section 18 for extractingenergy from the combustion gases. The engine 10 also comprises anauxiliary or accessory gearbox (AGB) 20 on which are located mechanicaland electrical systems, such as fuel pumps, oil pumps, generators and astarter/generator. The main rotating parts of the engine 10 areconnected in two subgroups, the low pressure (LP) spool and the highpressure (HP) spool, which are coaxially disposed. In use, the engine 10is started by the starter which is mechanically connected to the HPspool using a set of gears and a tower shaft 22. Once the desired HPspool speed is reached, fuel is provided into the combustor 16 and isignited to start or “light” the engine 10.

When the engine 10 is mounted on an airplane, in the unlikely event of aflame out or engine shutdown, dynamic pressure due to forward speed ofthe airplane creates a windmill effect to spin the LP and HP spools.This spinning is then further assisted by the starter to spin the HPspool up to the starting speed so that relight can successfully occur.In other arrangements, a shaft power transfer arrangement is provided totransfer windmilling energy from the LP spool to the HP spool to assistacceleration of the HP spool to relight speed. However, there is acontinuing need for simpler and better systems.

SUMMARY

In one aspect, the present invention provides a method for in-flightrelighting a turbofan engine of an aircraft, the engine having at leasttwo shafts, one of which is a high-pressure shaft mounted to ahigh-pressure compressor and a high-pressure turbine, the high-pressureshaft drivingly connected to an accessory load, the method comprisingthe steps of: disconnecting the accessory load from the high-pressureshaft to substantially eliminate a parasitic drag load on thehigh-pressure shaft; permitting ram air to rotate the high pressureshaft; and relighting the engine.

In another aspect, the present invention provides a method for in-flightrelighting an aircraft turbofan engine, the engine having at least twoshafts, one of which is a high-pressure shaft mounted to a high-pressurecompressor, a high-pressure turbine and an electrical generator, thegenerator electrically driving an accessory load, the method comprisingthe steps of: determining the presence of an engine-out condition of theengine; using the generator to reduce the rate of rotation of thehigh-pressure shaft to a desired rate within a relight envelope; andrelighting the engine.

In another aspect, the present invention provides a method for in-flightrelighting an aircraft accessory gearboxless turbofan engine, the enginehaving at least two shafts, one of which is a high-pressure shaftmounted to a high-pressure compressor, a high-pressure turbine and aconcentrically-mounted electrical generator, the generator electricallydriving an accessory load, the method comprising the steps of: usingexclusively ram air through the engine to rotate the high-pressureshaft; and then relighting the engine.

In another aspect, the invention provides a method of relighting a gasturbine engine of a fixed-wing aircraft after an in-flight engine-outcondition, the engine having at least one electromagnetic bearingapparatus and at least a bladed propulsor mounted to a first shaft and acompressor and turbine mounted to a second shaft, the first shaftdrivingly connected to an electric generator, the method comprising thesteps of: using windmill rotation of the bladed propulsor to drive thegenerator; using electricity from the windmill-driven generator toprovide power to the electromagnetic bearing apparatus; and relightingthe engine.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present method, and to show moreclearly how it may be carried into effect, reference will now be made byway of example to the accompanying figures, in which:

FIG. 1 schematic view of a typical turbofan gas turbine engine accordingto the prior art;

FIG. 2 is a schematic side view of an example of a turbofan gas turbineengine for use with the present method; and

FIG. 3 is block diagram illustrating the present method.

DETAILED DESCRIPTION

FIG. 2 shows a turbofan gas turbine engine 20 which generally comprisesa low-pressure (LP) spool 21 supporting at least a fan and a turbine,and a concentric high-pressure (HP) spool 24 supporting at least acompressor and a turbine. An embedded or integrated generator orstarter/generator 22 is coaxially mounted on the HP spool 24 of theengine 20, and preferably a second generator or motor/generator 23 ismounted on the LP spool 21 of the engine 20. Starter-generator 22 may beoperated as a motor to light engine 20, and also preferably as agenerator to generate electricity, which a controller 26 may thenprovide in form suitable for driving accessories 28 such aselectrically-driven pumps and other engine and aircraft services.Generator 23 may be used likewise to generate electricity for controller26 to provide to accessories 28 (but are not necessarily the samecontroller or accessories/services as driven by generator 22), and if amotor/generator, may be used to selectively drive the LP spool 21.Consequently, the need for an accessory gearbox is obviated, and is thusnot present in engine 20. The design of engine 20 is not new, howeverthe present invention offers new functionality to the engine 20 toprovide improved in-flight relighting, as will now be described.

After a flame-out or other shut-down of engine 20 occurs requiring theengine to be relit, in-flight windmilling causes the LP spool 21 and HPspool 24 to rotate, which thus rotates starter-generator 22. Duringin-flight windmilling, controller 26 preferably partially or completelydisconnects or stops supplying electricity to accessories 28, so thereis substantially no electrical load drawn from starter-generator 22, andthus there is substantially no parasitic drag on the HP spool 24 causedby starter-generator 22. For example, in one embodiment shown in FIG. 3,a flame-out (or other engine-off) condition is initially detected by thecontroller 26, which controls the fuel and oil pumps 28. The controlleralso monitors electrical output from the generator(s), and includessuitable means to prevent power output to the aircraft electrical bus(also represented by 28) which does not meet the specificationrequirements—i.e. the controller 26 has control over whether thestarter-generator 22 is connected to the bus in the ‘generate’ and‘start’ modes. In a flame-out condition, an appropriate sensor signalsthe controller to stop the fuel pump from pumping fuel, and preferablyalso stops the oil pump, and the electrical output of thestarter-generator 22 is also disconnected from the aircraft bus. Thus,electromagnetic drag on the HP spool 24 is reduced, and preferablyeffectively eliminated. Consequently, unlike the prior art, theaccessories 28 are disconnected from the HP spool 24, preferably priorto relight.

Referring again to the engine 10 of FIG. 1, during in-flight windmillingAGB 20 remains drivingly connected to the HP spool, and thus a pluralityof gears and accessories continue to be driven by the HP spool, whichcreates a parasitic mechanical drag on the HP spool which tends todecelerate the HP spool windmilling speed. As previously described,another energy source is required to overcome this drag and acceleratethe engine to its relight speed. However, by disconnecting the load fromthe HP spool 24 of engine 20, the parasitic drag of the accessory systemis virtually eliminated and, in the right conditions, windmill speedalone becomes sufficient to spin the HP spool 24 at a desired startingspeed, using only aircraft attitude if necessary to control windmillspeed. Another external power source is not required, therebysimplifying the engine system. This greatly facilitates relighting ofthe engine 20 by extending the in flight relight envelope of the engine.

Therefore, the windmilling effect of ram air though the high spool maybe used to rotate the engine to relight speed, particularly in verysmall turbofans having low inertia. Thus relight is achieved bydisconnecting accessories and then using windmilling power, preferablyalone and without the input of additional rotation energy from thestarter-generator 22, or any other power transfer mechanism, to increasethe speed of the HP spool.

In fact, conversely to the prior art, in some situations such as whendescending rapidly on flame out conditions, the rotor may tend to spintoo quickly, and thus prevent optimum relight conditions (e.g. lean blowout may occur if there is too much speed at the low fuel flows generallydesired for starting), adjustable “drag” may be provided to the highrotor, e.g. by providing a braking force to slow the HP spool speeddown. In one approach, this is achieved by operating thestarter/generator 22 as a sort of electromagnetic brake, for example bycontrolling the current of the starter-generator via the controller 26.In another aspect, a mechanical braking arrangement may be employed toretard spool rotation. This may be used to put an upper limit onwindmill speed under conditions requiring a specific relight speed,without requiring the pilot to set a different decent rate than wasrequired for other reasons (for example, in the case where both enginesflame out, descending to an altitude where there is air to breathe isoften high on the pilot's list of priorities). Thus, controlling thewindmill speed to an optimum value for relight, whether increasing ordecreasing the rotor speed as necessary, is available with the presentconcept.

In another aspect of the present invention, in the case of flame-out,generator 23 may provide self-contained back-up electrical to powerduring windmilling to a magnetic bearings power system (indicated asamong the elements of 28) to support the required shafts or spoolsduring power-out situations. The LP spool generator does not induceparasitic drag on the HP spool, and thus no hamper relighting of the HPspool.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that other changes may also be made to theembodiments described without departing from the scope of the inventiondisclosed. For instance, the starter-generator can be any suitabledesign, and may in fact be provided by two different units (e.g.separate starter and generator). Although it is desirable to adjustparasitic drag (e.g. by disconnecting accessories and/or reducing rotorspeed) prior to commencing relight procedures, the operations may beperformed in any desired order. Although electrically disconnecting ofthe HP spool from accessory drive systems is preferred, any suitableselectively operable disconnect system may be employed. Still othermodifications may be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

What is claimed is:
 1. A method for in-flight relighting an aircraftturbofan engine, the engine having at least two shafts, one of which isa high-pressure shaft mounted to a high-pressure compressor, ahigh-pressure turbine and an electrical generator, the generatorelectrically driving an accessory load, the method comprising:determining the presence of an engine-out condition of the engine;controlling a current of the generator to reduce the rate of rotation ofthe high pressure shaft to a desired rate within a relight envelope; andrelighting the engine.
 2. The method of claim 1, wherein current isprovided to the generator to cause the generator to apply a brakingforce on the shaft.